1. Field of the Invention
This invention relates to an integrated circuit for driving a liquid crystal device, etc.
2. Description of the Related Art
FIG. 3 shows an example of a liquid crystal device driving integrated circuit (IC). The IC 20 includes a bidirectional shift register 21, a latch unit 22, a level shifter 23, a driver unit 24, and logic circuits 25a and 25b. The IC 20 includes pins of DI, DO, SHL, CL1, CL2, M, Y.sub.1 -Y.sub.80, V, V.sub.CC, GND, and V.sub.EE. The IC 20 provides an 80-bit output specification.
The bidirectional shift register 21 shifts bits of serial data at the timing synchronized with a shift clock signal supplied via the CL2 pin from an external device. The serial data whose bits are to be shifted is supplied via the DI pin and logic circuit 25a from an external device. The logic circuit 25a changes the serial data supply destination in response to a direction selection signal supplied via the SHL pin from an external device. The bidirectional shift register 21 changes the bit shift direction in response to the direction selection signal. For example, if the direction selection signal has a value indicating bit shift from right to left in FIG. 3, serial data is supplied to the bidirectional shift register 21 from right and bits of the serial data are shifted from right to left. In contrast, if the selection signal has a value indicating bit shift from left to right in FIG. 3, serial data is supplied to the bidirectional shift register 21 from left and bits of the serial data are shifted from left to right.
The bidirectional shift register 21 consists of 80 bits. Therefore, if serial data exceeding 80 bits is supplied, the bidirectional shift register 21 overflows. The overflow serial data is output via the logic circuit 25b and DO pin to the following circuit, such as another IC.
The latch unit 22, which consists of 80 bits, latches data stored in the bidirectional shift register 21 at the timing synchronized with a latch clock signal supplied via the CL1 from the external device. The serial data input via the DI pin is converted into parallel data by bit shifting by the bidirectional shift register 21 and latching by the latch unit 22. The level shifter 23 shifts the level of the parallel data for supply to the driver unit 24. Then, the driver unit 24 converts the parallel data whose level is shifted into AC drive output signals and outputs the resultant signals through the Y.sub.1 -Y.sub.80 pins to liquid crystal elements. At the conversion, the driver unit 24 inputs a signal for AC conversion from the M pin and drive voltages V.sub.1 -V.sub.4 from the V pin and uses them for conversion. 80 liquid crystal elements (not shown) are disposed.
The V.sub.CC, GND, and V.sub.EE pins are IC positive power supply, ground, and negative power supply pins respectively.
To cascade ICs 20 shown in FIG. 3, the DO pin of the preceding IC 20 should be connected to the DI pin of the following IC 20. Then, for example, if eight ICs 20 are cascaded, drive output signals of 80.times.8=640 dots can be provided.
If the number of dots is not an integer multiple of 80 dots, the following methods are available: Method A by which a necessary number of dots are charged equally to each of ICs 20 and method B by which each device of the ICs 20 is used as 80-dot output and the remainder given by dividing the total of dots by 80 is charged to one IC 20.
Assume that the liquid crystal device 26 provides 24.times.24 dots=576 dots for static character display. Method A requires the circuit configuration as shown in FIG. 4, for example. Method B requires the circuit configuration as shown in FIG. 5, for example.
For method A, because 72.times.8=576, each of eight ICs 20 may be used as a 72-bit output IC without using output pins Y.sub.73-80 (eight bits) of each IC 20. On the other hand, for method B, because 80.times.7 +16=576, all of seven ICs 20 may be used 80-bit output ICs with the eighth IC 20 as a 16-bit output IC.
To display a character made up of 16.times.16=256 dots, if method A is used, each of four ICs 20 may be used as a 64-bit output IC; if method B is used, three ICs 20 may be used as 80-bit output ICs with the fourth device of IC 20 as a 16-bit output IC. Likewise, to display a character made up of 32.times.32=1024 dots, if method A is used, sixteen ICs 20 may be used as 64-bit output ICs; if method B is used, tweleve ICs 20 are used as 80-bit output ICs and eleven ICs 20 are used as 64-bit output ICs. Further, to display a character made up of 48.times.48=2304 dots, if method A is used, thirty-two ICs 20 are used as 72-bit output ICs; if method B is used, twenty-eight ICs 20 are used as 80-bit output ICs and one IC 20 is used as a 64-bit output IC.
These methods are not adequate.
First, when the necessary total of output bits cannot be divided by the number of output bits of a single IC 20 (in the example given above, 80 bits), if method A is used, some of the Y pins are not used to drive the liquid crystal device 26. For example, if only 72 bits of 80 Y pins (80 bits) are used, eight Y pins (eight bits) remain unused. Therefore, data input through the DI pin of the IC 20 at the first stage must be mixed with dummy data in such a manner that 8-bit dummy data is input before significant 72-bit serial data is input. The process of mixing serial data with dummy data is very complicated. Although it is possible to change the output bit specification of the IC 20 according to the number of dots of the liquid crystal device 20 to be driven, it is not economical to provide many types of output bit specifications of the IC 20 so as to cover all of various kinds of specifications of the number of dots of the liquid crystal device 26.
Method B does not introduce such a problem. However, if the necessary total of output bits cannot be divided by the number of output bits of a single IC 20, only some of the Y pins of the IC 20 at the last stage are used unlike other ICs 20 at preceding stages. This means that the IC 20 at the last stage and other ICs 20 must differ in wiring pattern between the IC 20 and the liquid crystal device 26. Therefore, special consideration must be given to the last IC 20 to assemble circuitry containing a cascade of the ICs 20. Thus, method B introduces a problem of an increase in the number of assembly steps.